1. Field of the Invention
The present invention relates to a unitary module for a planar flexible pivot as well as to a planar flexible pivot formed by a stack of unitary modules. It relates especially to a method for the manufacture of said unitary module and to assemblies and applications of said planar flexible pivots.
The invention can be applied to special advantage in the field of elastic joints, especially cross-band pivots.
2. Description of the Prior Art
The technology of elastic joints conventionally teaches that, for systems of guidance with low amplitude of displacement, it is advantageous to use a thin and elongated spring band (hereinafter called a band) that is embedded at one end and subjected to forces at the other end, with the property of easily lending itself to a flexing motion in the direction perpendicular to the band while at the same time remaining rigid in the parallel directions.
When the strains are calculated so that they remain well within the elastic limits and the buckling loads, a perfectly precise and reproducible device is obtained, free from play, friction or wear and tear, and with no phenomena of jamming. The reproducibility is limited only by the residual hysteresis applied to the non-compensated effects due to the variation of the loads and the finite rigidity of the bands. It must also be added that no lubrication is necessary.
Within the limits of static deflection mentioned, elastic joints thus have major advantages over conventional kinematic joints such as blocks, slides, bearings, etc. The permitted load limits are all the higher as the deflection required is low and as the thickness of the elastic joints may be great.
The principle of a one-band flexible pivot is shown in FIG. 1 which gives a side view of a flexible band 10 with a length L, one end 10a of which is embedded. When a torque T is applied to the free end 10b of the band 10, this band gets curved and said free end 10b shifts in pivoting roughly about the axis passing through the center 11 of the band 10 at rest, thus defining a flexible-band pivot.
However, a one-band pivot is not stable under the action of a load perpendicular to the plane of the band. To overcome with this drawback, the cross-band pivot has been designed. An example of such a pivot is shown in the view in perspective of FIG. 2. In this example, the two bands 10, 20 are crossed at right angles, their ends being fixed by screws to supports 1, 2.
In order to make the flexible cross-band pivot less sensitive to the effect of off-plane loads, it is possible to use two pairs of crossed bands located symmetrically at a certain axial distance from each other. Thus, a great traversal rigidity and high resistance to unwanted action is obtained. Furthermore, the instantaneous center of rotation is practically fixed up to pivoting angles of 10.degree..
It has also been sought to compensate for the elastic reaction of rotation of the flexible-band pivots-by subjecting the bands to a traction force, the elastic energy being exchanged between the bands and draw springs. Among the pivots of this type, there is a known device with three bands positioned at 120.degree. which furthermore has the advantage of enabling, in principle, a rotation about an invariable axis as a function of the pivoting deflection.
However, the known cross-band flexible pivots have a certain number of drawbacks.
Indeed, all the pivots mentioned here above are based on the concept of distinct separate bands that have to be screwed in, bonded or soldered to supports. In every case, the band/support joining area is subjected to a set of stresses due to the combined effects of the rotational deflection of the bands, the external load, especially the axial load, and internal hyperstatic stresses (membrane effects) acting on the pivot. This state of mechanical stress is a major factor in the loosening or disconnecting of assemblies and may lead to the breakage of the soldered parts through cyclical fatigue.
Furthermore, the position of the pivoting axis varies with the rotation, which corresponds to the lack of balance of the pivot. In addition to the systematic variation provided for by theory, it is also possible to observe variations that are reversible in varying degrees and random due to the effects of membrane hyperstaticity. This last-mentioned form of behaviour also depends on clamping strains when the pivot-fastening supports are cylindrical.
Finally, it is difficult to achieve the automation of the assembly process as also the miniaturization of the separate-band pivots. This makes the cost of the small-sized pivots comparatively high.